As well known, in an internal combustion engine, the combustion process and pollutant emissions are worst at start-up, when the internal combustion engine is still cold.
In order to reduce the warm-up time necessary to make the internal combustion engine fully effective, i.e. in order to speed up the warm-up of the internal combustion engine (ICE), is well known in the art to divert the coolant flowing in the cooling loop circuit back to the engine, without allowing the coolant to reach the relevant radiator, till the coolant temperature, and hence the engine, reaches a target temperature, that is around 90° C.
More in detail, a cooling circuit of an internal combustion engine usually comprises a circuit loop for the coolant (e.g. water), connecting the ICE to the radiator, and a coolant pump. The cooling circuit is usually provided with a by-pass conduit allowing to exclude the radiator, as well as with a thermostat that allows the coolant to divert back to internal combustion engine, and hence that prevents the coolant from flowing through the radiator, till the coolant temperature reaches said target temperature, that is generally about 90° C.
Such a thermostat, that is a three-way valve actuated by temperature, is usually a wax-type thermostat, in which a thermostat body comprises two inlets, coming from the engine via the by-pass conduit, and from the radiator respectively, and one outlet directed to the engine, as well as a closure element integral to a piston that is slidingly actuated by wax contained in a wax chamber, which in turn is thermally connected to the coolant flowing through the thermostat body, and in particular to the coolant coming from the aforesaid inlet from the engine.
In particular, the closure element of thermostats in a cooling system of an internal combustion engine opens or closes the pathway between the inlet from the radiator and the outlet to the engine, while the pathway between the inlet from engine and the outlet to engine remains usually open.
The thermostat includes a return spring biasing the closure element towards its closing position and, since the piston integral to the closure element of the thermostat is mechanically linked to the wax in the wax chamber, when the wax increases its volume (due to the raising of the coolant temperature through the thermostat body) the piston is pushed by the same wax, in contrast to the spring action, to move the closure element until it reaches its opening position.
Usually wax is chosen in such a way that when the target temperature of the coolant is almost reached, it changes its status from solid to liquid, expanding its volume as well, and thus pushing the piston in contrast to the spring and opening the pathway between the radiator inlet and the engine outlet of the thermostat.
To improve the behaviour of the thermostat, is well known to provide it with heating means, generally constituted by some electrical heater regulated by the ECU (Electronic Control Unit) of the power-train, heating the wax in said wax chamber.
The presence of an electrical heater for heating the wax even when the coolant has not yet reached the target temperature, thus opening the pathway between the radiator and the ICE when the ECU decides it is appropriate, allows a more stable behaviour of the thermostat as well as a quick cooling of the engine when the heat produced from this latter may cause overheating problems.
Such a thermostats for the cooling system of an internal combustion engine suffers from the problem that when the closure element opens for the first time the pathway from the radiator to the engine, the coolant coming from the radiator is colder (till ΔT°>110° C.) than the coolant coming from engine and thus it may cause a quick falling of the wax temperature in the wax chamber, such a way the wax reduces its volume and the closure element quickly moves towards its fully close position, biased by said return spring.
At this point, the higher temperature of the coolant coming from the engine causes again a wax volume increase, such a way the thermostat closure element is pushed again towards its fully open position, allowing the low temperature coolant coming from radiator to reduce once more the wax temperature, etc.
Such an instable behaviour of the thermostat causes temperature fluctuations till a thermal steady state is reached.
It is therefore desirable to keep the closure element open during the starting transient when coolant with low temperature coming from the radiator starts to flow within the thermostat, in this way reducing the wax temperature in the wax chamber and thus inducing the closure element to close.
On the other hand, it should also be noticed that heating the wax with the aforesaid electrical heater within the wax chamber, as provided in a thermostat of the prior art, in order to keep open the thermostat, may lead, during the aforesaid starting transient, to the overheating of the wax in the chamber or to an insufficient opening of the thermostat closure element.
Thus, it is an aim of the present invention to provide a new wax-type thermostat for the cooling system of an internal combustion engine that does not show the problems described above, and hence that allows the closure element of the thermostat to be kept open during the cooling starting transient of the engine.
Another aim of this invention is to provide a new wax-type thermostat for the cooling system of an internal combustion engine that prevents thermal instability in the cooling system and that proves to be effective to quickly keep the coolant at a target steady temperature.